Device for hot dip coating metal strands

ABSTRACT

The invention relates to a device for hot dip coating metal strand ( 1 ), particularly strip steel, in which the metal strand ( 1 ) can be vertically guided through a reservoir ( 3 ), which accommodates the molten coating metal ( 2 ), and though a guide channel ( 4 ) connected upstream therefrom. An electromagnetic inductor ( 5 ) is mounted in the area of the guide channel ( 4 ) and in order to retain the coating metal ( 2 ) inside the reservoir ( 3 ), can induce induction currents in the coating metal ( 2 ) by an electromagnetic traveling field. While interacting with the electromagnetic traveling field, the induction currents exert an electromagnetic force. The inductor ( 5 ) has at least two main coils ( 6 ) that are arranged in succession in movement direction (X) of the metal strand ( 1 ), and has at least two correction coils ( 7 ) for controlling the position of the metal strand ( 1 ) inside the guide channel ( 4 ) in direction (N), which is normal to the surface of the metal strand ( 1 ). These correction coils are also arranged in succession in movement direction (X) of the metal strand ( 1 ). In order to improve the efficiency of the control of the metal strip inside the guide channel, the invention provides that at least a portion of the correction coils ( 7 ), when viewed in movement direction (X) of the metal strand ( 1 ), are arranged so that they are offset with regard to one another perpendicular to movement direction (X) and perpendicular to direction (N) that is normal to the surface of the metal strand ( 1 ).

The invention concerns a device for the hot dip coating of metalstrands, especially steel strip, in which the metal strand can be passedvertically through a tank that contains the molten coating metal andthrough an upstream guide channel. In the area of the guide channel, anelectromagnetic inductor is installed, which induces induction currentsin the coating metal for holding back the coating metal in the tank bymeans of an electromagnetic traveling field. The induction currentsinteract with the electromagnetic traveling field to exert anelectromagnetic force. The inductor has at least two main coils, whichare arranged in succession in the direction of movement of the metalstrand, and at least two correction coils, which serve to control theposition of the metal strand in the guide channel in the directionnormal to the surface of the metal strand and are also arranged insuccession in the direction of movement of the metal strand.

Conventional metal dip coating systems for metal strip have ahigh-maintenance part, namely, the coating tank and the fixtures itcontains. Before being coated, the surfaces of the metal strip to becoated must be cleaned of residual oxide and activated to allow bondingwith the coating metal. For this reason, the surfaces of the strip aresubjected to a heat treatment in a reducing atmosphere before they arecoated. Since the oxide coatings are first removed by chemical orabrasive methods, the reducing heat treatment activates the surfaces, sothat after the heat treatment, they are present in pure metallic form.

However, the activation of the strip surface increases the affinity ofthe strip surface for the surrounding atmospheric oxygen. To prevent thesurface of the strip from being re-exposed to atmospheric oxygen beforethe coating process, the strip is introduced into the hot dip coatingbath from above in a dipping snout. Since the coating metal is presentin the molten state, and since one would like to utilize gravitytogether with blowing devices to adjust the coating thickness, but thesubsequent processes prohibit strip contact until the coating metal hascompletely solidified, the strip must be deflected in the verticaldirection in the coating tank. This is accomplished with a roller thatruns in the molten metal. This roller is subject to strong wear by themolten coating metal and is the cause of shutdowns and thus loss ofproduction.

The desired low coating thicknesses of the coating metal, which vary inthe micrometer range, place high demands on the quality of the stripsurface. This means that the surfaces of the strip-guiding rollers mustalso be of high quality. Problems with these surfaces generally lead todefects in the surface of the strip. This is a further cause of frequentplant shutdowns.

In addition, previous hot dip coating systems have limiting values intheir coating rates. These limiting values are related to the operationof the stripping jets, to the cooling processes of the metal strippassing through the system, and to the heat process for adjusting alloycoatings in the coating metal. As a result, the maximum rate isgenerally limited, and certain types of metal strip cannot be conveyedat the plant's maximum possible rate.

During the hot dip coating process, alloying operations for the bondingof the coating metal to the surface of the strip are carried out. Theproperties and thicknesses of the alloy coatings that form are stronglydependent on the temperature in the coating tank. For this reason, inmany coating operations, although, of course, the coating metal must bemaintained in a liquid state, the temperatures may not exceed certainlimits. This conflicts with the desired effect of stripping the coatingmetal to adjust a certain coating thickness, since the viscosity of thecoating metal necessary for the stripping operation increases withdecreasing temperature and thus complicates the stripping operation.

To avoid the problems associated with rollers running in the moltencoating metal, approaches have been proposed, in which a coating tank isused that is open at the bottom and has a guide channel in its lowersection for guiding the strip vertically upward, and in which anelectromagnetic seal is used to seal the open bottom of the tank. Theproduction of the electromagnetic seal involves the use ofelectromagnetic inductors, which operate with electromagneticalternating or traveling fields that seal the coating tank at the bottomby means of a repelling, pumping, or constricting effect.

A solution of this type is described, for example, in EP 0 673 444 B1.The solutions described in WO 96/03533 and JP 50[1975]-86446 alsoprovide for an electromagnetic seal for sealing the coating tank at thebottom.

Although this allows the coating of nonferromagnetic metal strip,problems arise in the coating of steel strip that is essentiallyferromagnetic, because the strip is drawn to the walls of the channel bythe ferromagnetism in the electromagnetic seals, and this damages thesurface of the strip. Another problem that arises is that the coatingmetal is unacceptably heated by the inductive fields.

An unstable equilibrium exists with respect to the position of theferromagnetic steel strip passing through the guide channel between twotraveling-field inductors. The sum of the forces of magnetic attractionacting on the strip is zero only in the center of the guide channel. Assoon as the steel strip is deflected from its center position, it drawscloser to one of the two inductors and moves farther away from the otherinductor. The reasons for this type of deflection may be simple flatnessdefects of the strip. Defects of this type include any type of stripwaviness in the direction of strip flow, viewed over the width of thestrip (center buckles, quarter buckles, edge waviness, flutter, twist,crossbow, S-shape, etc.). The magnetic induction, which is responsiblefor the magnetic attraction, decreases in field strength with increasingdistance from the inductor according to an exponential function.Therefore, the force of attraction similarly decreases with the squareof the induction field strength with increasing distance from theinductor. This means that when the strip is deflected in one direction,the force of attraction to one inductor increases exponentially, whilethe restoring force by the other inductor decreases exponentially. Botheffects intensify by themselves, so that the equilibrium is unstable.

DE 195 35 854 A1 and DE 100 14 867 A1 offer approaches to the solutionof this problem, i.e., the problem of more precise position control ofthe metal strand in the guide channel. According to the conceptsdisclosed there, the coils for inducing the electromagnetic travelingfield are supplemented by correction coils, which are connected to anautomatic control system and see to it that when the metal stripdeviates from its center position, it is brought back into thisposition.

In these previously known approaches to a solution of this problem, itwas found to be a disadvantage that the automatic control of the metalstrip for keeping the strip in the center of the guide channel becomesdifficult due to the fact that destructive interference of the fieldssometimes occurs due to the superimposing of the magnetic fields of themain coils and correction coils, and therefore efficient restoration ofthe metal strip to the center of the guide channel becomes difficult orimpossible. An analysis of the resisting forces of the steel striprevealed that with decreasing strip thickness, which conforms to thepresent trend, the inherent stiffness of the steel strip decreases tothe extent that the strip can offer very little resistance todeformation by the magnetic field of the inductors. A problem in thisregard is the large unsupported length between the lower guide rollerbelow the guide channel and the upper guide roller above the coatingbath, which can be well above 20 m in a production plant. This increasesthe need for efficient position control of the metal strip in the guidechannel, which is difficult due to the conditions noted above.

Therefore, the objective of the invention is to further develop a devicefor the hot dip coating of metal strands of the type specified at thebeginning in such a way that the specified disadvantages are overcome.In particular, it should be possible to keep the metal strip in thecenter of the guide channel in an effective way.

In accordance with the invention, this objective is achieved byarranging at least some of the correction coils, as viewed in thedirection of movement of the metal strand, in a staggered fashionrelative to one another perpendicular to the direction of movement andperpendicular to the direction normal to the surface of the metal strip.

The correction coils, as viewed in the direction of movement of themetal strip, are preferably arranged in at least two rows, andpreferably six rows. In addition, each row can have at least twocorrection coils. Furthermore, it is advantageous to provide for thecenter of a correction coil to be arranged in a following row, as viewedin the direction of movement of the metal strand, exactly between twocenters of the correction coils of the preceding row.

The advantage obtained with the refinement in accordance with theinvention is that, due to the staggered arrangement of the correctioncoils from row to row (as viewed in the direction of movement of themetal strand), the magnetic fields of traveling-field coils for sealingthe guide channel and the magnetic fields of the correction coils forcontrolling the position of the strip in the guide channel aresuperimposed on one another to form a common field, which both seals andcontrols. The invention avoids the problem of destructive interferenceof the fields due to mutually neutralizing magnetic fields at theboundaries of the correction coils in a row, which otherwise would nolonger allow an influence to be exerted on the metal strip in the guidechannel for the purpose of controlling its position.

In the arrangement provided for in accordance with the invention, theinduction fields are superimposed on one another, and the unwantedeffect of destructive interference of the fields on the side iscompensated by the correction coil located below it in a staggeredposition. On the lower side of the inductors, the effect is no longer aproblem, since the controlled region for the column of liquid metal islocated in the upper half of the guide channel and therefore no longerhas an interfering effect in this area.

In accordance with a further development, it is provided that at leastone correction coil, as viewed in the direction of movement of the metalstrand, is arranged at the same height as each main coil. Furthermore,it can be provided that the electromagnetic inductor has a number ofgrooves that run perpendicularly to the direction of movement of themetal strand and perpendicularly to the normal direction for holding themain coils and correction coils. In this regard, it can beadvantageously provided that at least a part of at least one main coiland at least one correction coil is mounted in each groove. Moreover, ithas been found to be advantageous for the part of the correction coilmounted in the groove to be mounted closer to the metal strand than thegiven part of the main coil.

Special importance is attached to the supplying of both the main coilsand the correction coils with alternating current. For this purpose,means are preferably provided by which the main coils can be suppliedwith three-phase alternating current. It is especially advantageous toinstall a total of six main coils arranged in succession in thedirection of movement of the metal strand (i.e., six rows), which aresupplied with three-phase current that differs in phase successively by60°.

Furthermore, it is proposed that means be used by which the correctioncoils are supplied with an alternating current that has the same phaseas the current with which the locally adjacent main coil is operated.

Current supply with pulse synchronization over optical waveguides canpreferably be used for the in-phase supplying of the main coils andcorrection coils.

This type of refinement of the invention makes it possible to operatethe correction coils in phase with the traveling field. Usually threephases of a rotating field are used for the traveling-field inductors;for the correction coils, the respective single phase of the main coilin front of which the correction coil is located is sufficient. For thepower supply of the two inductors on either side of the metal strand,three-phase variable-frequency inverters can be used for the travelingfield; single-phase variable-frequency inverters are sufficient for thecorrection coils, specifically, one for each correction coil. Thesynchronization of the individual variable-frequency inverters is ofessential importance in this regard. This can be accomplished in anespecially simple way by the aforementioned pulse synchronization overoptical waveguides, which is especially advisable due to the strongmagnetic fields and their stray fields.

The position of the running steel strip can be detected by inductionfield sensors, which are operated with a weak measuring field ofpreferably high frequency. For this purpose, a voltage of higherfrequency with low power is superposed on the traveling-field coils. Thehigher-frequency voltage has no effect on the seal; in the same way,this does not produce any heating of the coating metal or steel strip.The higher-frequency induction can be filtered out from the powerfulsignal of the normal seal and then yields a signal proportional to thedistance from the sensor. The position of the strip in the guide channelcan be detected and controlled with this signal.

Studies on the inherent stiffness of the metal strand revealed adefinite improvement of the controllability of the metal strip with theproposed refinement of the correction coils. The strip thus no longerhas long unsupported lengths in the area of the inductors, and it thushas sufficient inherent stiffness to allow its position to be controlledas it passes through the guide channel.

An embodiment of the invention is illustrated in the drawings.

FIG. 1 shows a schematic representation of a hot dip coating tank with ametal strand being guided through it.

FIG. 2 shows the front view of an electromagnetic inductor, which isinstalled at the bottom of the hot dip coating tank.

FIG. 3 shows the side view of the electromagnetic inductor correspondingto FIG. 2.

FIG. 4 shows the phase sequence of the electromagnetic traveling fieldinduced by the electromagnetic inductor.

FIG. 1 shows the principle of the hot dip coating of a metal strand 1,especially a steel strip. The metal strand 1 that is to be coated entersthe guide channel 4 of the coating system vertically from below. Theguide channel 4 forms the lower end of a tank 3, which is filled withmolten coating metal 2. The metal strand 1 is guided vertically upwardin direction of movement X. To prevent the molten coating metal 2 frombeing able to run out of the tank 3, an electromagnetic inductor isinstalled in the area of the guide channel 4. It consists of two halves5 a and 5 b, which are installed on either side of the metal strand 1.In the electromagnetic inductor 5, an electromagnetic traveling field isinduced, which holds the molten coating metal 2 in the tank 3 and thusprevents it from running out.

The exact design of the electromagnetic inductor can be seen in FIGS. 2and 3, which show only one of the two symmetrically designed inductors 5a, 5 b, which are installed on either side of the metal strand 1. As isshown in FIG. 2, the metal strand 1 moves upward past the inductor 5 ain the direction of movement X. The inductor 5 a is equipped with atotal of six main coils 6 for induction of the electromagnetic travelingfield. The main coils extend over the entire width of the inductor 5 a(see FIG. 3). The main coils 6 are mounted in grooves 10, which areincorporated in the metallic foundation of the inductor 5 a. The currentdirections are indicated on the right side of FIG. 2 for a total of fiveline sections of the main coils 6, as they either emerge from the planeof the drawing or enter the plane of the drawing.

To allow the metal strand 1 to be held exactly in the center of theguide channel 4 in the direction N normal to the surface of the strand 1(see FIG. 2 and FIG. 3) without hitting the inductors 5 a, 5 b,correction coils 7 are mounted in the inductors 5 a, 5 b. As especiallyFIG. 3 shows, several correction coils 7 are positioned side by side ineach of the total of six rows 8′, 8″, 8′″, 8″″, 8′″″, 8″″″. The maincoil 6, which extends over the entire width of the inductor 5 a, andseveral correction coils 7, which are positioned side by side, aremounted in two adjacent grooves 10.

As FIG. 3 shows, the coils are arranged in such a way that thecorrection coils 7 of two successive rows 8′, 8″, 8′″, 8″″, 8′″″, 8″″″are staggered relative to one another. The center of the correctioncoils is labeled with reference number 9. As is apparent from the bottomright of FIG. 3, the distances a and b are the same and indicate theamount of offset of the correction coils 7 relative to one another. Thisrefinement ensures that the magnetic fields induced by the correctioncoils 7, which control the position of the metal strand 1 in the guidechannel 4, cannot destructively interfere with one other. This allowsefficient position control.

FIG. 4 shows the phase sequence of the three-phase current, as it existsin the six main coils 6 shown in the drawings. The three phases arelabeled R, S, and T. The phase sequence is R, -T, S, -R, T, -S.

Each correction coil 7 must be driven with the same phase that ispresent in the main coil 6 in front of which the given correction coil 7is positioned. The main coils 6 for the induction of the traveling fieldare thus driven with three phases of a rotating field, while each of thecorrection coils 7 is supplied with only one phase. The supplying of thecoils 6 and 7 with phase-exact directional current is realized by meansof suitable and sufficiently well-known variable-frequency inverters,which must be suitably synchronized, for which purpose especially pulsesynchronization over optical waveguides is well suited.

LIST OF REFERENCE NUMBERS

-   1 metal strand (steel strip)-   2 coating metal-   3 tank-   4 guide channel-   5, 5 a, 5 b electromagnetic inductor-   6 main coil-   7 correction coil-   8′, 8″, 8′″, 8″″, 8′″″, 8″″″ rows-   9 center of a correction coil 7-   10 groove-   X direction of movement-   N normal direction-   a distance between the centers 9-   b distance between the centers 9-   R phase of the three-phase current-   S phase of the three-phase current-   T phase of the three-phase current

1. Device for the hot dip coating of metal strand (1), especially steelstrip, in which the metal strand (1) can be passed vertically through atank (3) that contains the molten coating metal (2) and through anupstream guide channel (4), wherein, in the area of the guide channel(4), an electromagnetic inductor (5) is installed, which can induceinduction currents in the coating metal (2) for holding back the coatingmetal (2) in the tank (3) by means of an electromagnetic travelingfield, which induction currents interact with the electromagnetictraveling field to exert an electromagnetic force, and wherein theinductor (5) has at least two main coils (6), which are arranged insuccession in the direction of movement (X) of the metal strand (1), andat least two correction coils (7), which serve to control the positionof the metal strand (1) in the guide channel (4) in the direction (N)normal to the surface of the metal strand (1) and are also arranged insuccession in the direction of movement (X) of the metal strand (1),wherein at least some of the correction coils (7), as viewed in thedirection of movement (X) of the metal strand (1), are arranged in astaggered fashion relative to one another perpendicular to the directionof movement (X) and perpendicular to the direction (N) normal to thesurface of the metal strip (1).
 2. Device in accordance with claim 1,wherein the correction coils (7), as viewed in the direction of movement(X) of the metal strand (1), are arranged in at least two rows (8′, 8″,8′″, 8″″, 8′″″, 8″″″).
 3. Device in accordance with claim 2, whereineach row (8′, 8″, 8′″, 8″″, 8′″″, 8″″″) has at least two correctioncoils (7).
 4. Device in accordance with claim 3, wherein the center (9)of a correction coil (7) in a following row (8″), as viewed in thedirection of movement (X) of the metal strand (1), is arranged betweentwo centers (9) of the correction coils (7) of the preceding row (8′).5. Device in accordance with claim 1, wherein at least one correctioncoil (7), as viewed in the direction of movement (X) of the metal strand(1), is arranged at the same height as each main coil (6).
 6. Device inaccordance with claim 1, wherein the electromagnetic inductor (5) has anumber of grooves (10) that run perpendicularly to the direction ofmovement (X) of the metal strand (1) and perpendicularly to the normaldirection (N) for holding main coils (6) and correction coils (7). 7.Device in accordance with claim 6, wherein at least a part of at leastone main coil (6) and at least one correction coil (7) is mounted ineach groove (10).
 8. Device in accordance with claim 7, wherein the partof the correction coil (7) mounted in the groove (10) is mounted closerto the metal strand (1) than the given part of the main coil (6). 9.Device in accordance with claim 1, providing means for supplying themain coils (6) with three-phase alternating current.
 10. Device inaccordance with claim 9, wherein a total of six main coils (6) arrangedin succession in the direction of movement (X) of the metal strand (1),which are supplied with three-phase current that differs in phasesuccessively by 60°.
 11. Device in accordance with claim 9, providingmeans for supplying the correction coils (7) with an alternating currentthat has the same phase as the current supplied to the locally adjacentmain coil (6).
 12. Device in accordance with claim 11, wherein the meansfor supplying the main coils (6) and the correction coils (7) withalternating current has a device for pulse synchronization over opticalwaveguides.
 13. Device in accordance with claim 1, as viewed in thedirection of movement (X) of the metal strand (1), are arranged in sixrows (8′, 8″, 8′″, 8″″, 8′″″, 8″″″).